The present invention relates to a continuous backwashing filter apparatus for separating particulate matter from a fluid medium. The continuous backwashing feature enables continuous filtering of high volumes of a fluid medium containing gaseous matter or particles having a broad particle size distribution, including ultrafine particles having a particle size below, for example, 1 .mu.m.
Conventional filter arrangements have been known wherein a backwash device is provided proximate an inlet side of a filter element in the filter arrangement. For instance, U.S. Pat. No. 4,085,051, to Kaminsky et al, discloses a self-cleaning filter for separating solid particles from a liquid wherein a device for backwashing the filter element abuts the filter element and is rotated about a central shaft such that the filter element can be backwashed of any accumulated particles. However, the backwashing function requires a pressure differential between an inlet chamber and an outlet chamber of the filter apparatus due to the buildup of particulate matter on the inlet side of the filter element.
The creation of that necessary pressure differential requires the presence of a substantial amount of filtered fluid in the outlet chamber. Stated differently, filtered fluid must be present at an outlet side of the filter to provide a liquid medium passing in a reverse direction through the filter to transport the particulate matter off of the inlet side of the filter and into the backwashing device. However, such filtered fluid is not always present at the outlet side in the device of Kaminsky et al. When filtering fluids having particles of a broad particle size distribution, it is inevitable that the filter element will become clogged before there is a sufficient amount of filtered fluid in the outlet chamber to facilitate the backwashing function.
U.S. Pat. No. 3,635,348, to Carr, discloses an automatic self-cleaning strainer which provides a backwash device in abutted contact with an inlet side of a filter element in the strainer. The filter element is shaped like a truncated cone and the shape of the contacting surface of the backwash device is complimentary to the inlet side of the filter element. The backwash device is retractable along a central axis of the filter element to facilitate sequential radial movement of the backwash device within the filter element. This strainer apparatus cannot provide continuous backwashing due to the sequential action of the backwash device. As a result, the efficiency of the strainer apparatus is degraded.
The strainer of the Carr apparatus also suffers from the problem encountered in the filter apparatus of Kaminsky et al in that the backwashing function is contingent upon whether the outlet chamber is filled with filtered fluid, i.e., whether filtered fluid is present proximate an outlet side of the filter element. Accordingly, the strainer apparatus of Carr cannot filter fluid mediums having a broad particle size distribution including ultrafine particles, since it is inevitable that the filter element will become clogged before the outlet chamber is filled with filtered fluid to facilitate the backwashing function.
U.S. Pat. No. 3,640,395, to Kinney, discloses an automatic self-cleaning strainer having a plurality of filter stages including backwash devices in loose contact with an inlet side of the filter element of each filtering stage. Each of the backwashing devices is in communication with a hollow shaft which is open to the atmosphere. However, before the backwashing function will occur, filtered fluid must be present in the outlet chambers of each filtering stage. Therefore, the strainer apparatus cannot filter a fluid medium which contains large amounts of particulate matter including ultrafine particles, since the filter element in each stage will become clogged before the outlet chambers are filled with filtered fluid to facilitate the backwashing function.
U.S. Pat. No. 3,669,269, to Kinney, discloses a filter apparatus having a rotating cylindrical strainer element and a stationary backwash chamber in contact with an inlet side of the strainer element. However, in the device of Kinney '269, as in the other prior art devices, the outlet chamber within the strainer element must be substantially full of filtered fluid before the backwashing function will occur, which does not necessarily always happen. Thus, this strainer apparatus suffers from the same problems as discussed above.
The inventor's prior U.S. Pat. No. 5,128,029 (Herrmann '029) relates to a continuous backwashing filter apparatus with a filtered fluid retention plate. In this apparatus, a sliding shoe containing a slit contacts a perforated plate placed over a filter medium on the inlet side, and removes particles from the medium by backwashing with a reservoir of filtered liquid retained on a retention plate on the outlet side of the filter media.
Backwashing occurs because the pressure in the sliding shoe is less than in the outlet side of the filter. The retention plate is critical to the function of this apparatus, since it provides the constant supply of filtered fluid necessary for constant backwashing. This arrangement, while functional, has several drawbacks.
One drawback is the size limitation of the filter imposed by the sliding shoe/retention plate arrangement. The sliding shoe must be sealed from the unfiltered fluid on the inlet side. If this seal is broken, unfiltered fluid under high pressure will leak into the sliding shoe, and be removed with the backwash, essentially "short circuiting" the filter. This places severe limits on the maximum size and capacity of the filter. For example, filtration of a liquid having a high level of total suspended solids (TSS) with a filter media having a nominal pore size of 10 .mu.m or less results in very limited filter capacity. This is because a very high differential pressure is necessary to maintain a high flow rate under these conditions. Under such high pressure, the perforated plate above the filter media is forced against the sliding shoe and the backwash function is disabled.
If the size of the filter is increased to provide greater capacity, the contact area between the sliding shoe and the filter media can flex, resulting again in leakage and short circuiting of the filter. Likewise, installing two or more filters in parallel is generally difficult, since the structure of the backwashing function does not lend itself to parallel connection of multiple units.
A second drawback of the inventor's prior design is that, in some embodiments, the retention plate must be maintained horizontally level for proper backwashing, and hence installation of the backwashing filter apparatus is limited to an upright position.
Thus, despite the improvements in filtration afforded by the Herrmann '029 patent, there remains a need in the art for a filter apparatus which can filter very small particles at high capacity, can be installed in any position, and which can be easily expanded to provide further increases in capacity when necessary, and further eliminates the possiblity of leakage.
Sewage treatment is one area in which filtration of small particles at high capacity would be useful. In conventional biological treatment systems, particulates in the wastewater isolate the available oxygen necessary for microbe growth, resulting in delay or elimination of biological breakdown of waste material. An apparatus suitable for ultrafiltration at the high capacities needed for wastewater treatment would greatly expand the capacity and quality of discharge in existing treatment plants and/or reduce the size and complexity of equipment needed for treating a given quantity of sewage.